Most homeowners who are considering adding solar panels to their home have one specific question: how many panels do I need? While this is an important question, it's actually not the ideal starting point. The number of panels needed for any given home is easy to determine only after two other important questions have been answered: how much power generation is required, and how much output can be expected from each panel? At sea level, the total density of energy that reaches the ground from the sun is around 1000 W/m². This sounds like a lot (and it is!), but how much of that energy can be captured by any single panel depends on a variety of important factors.

What Is Solar Panel Efficiency?

The simplest explanation of panel efficiency is that it is the percentage of solar energy that a panel is capable of capturing and converting into electricity. If a square meter solar panel is 25% efficient and your home is located in an area that receives around the average of 1000 W/m², then you can expect the panel to output roughly 250W. Newer, higher-end panels tend to be more efficient than older, cheaper panels. Knowing the efficiency of the panels that you are purchasing is the first step to determining their overall output.

Efficiency and Output Can Be Deceiving

Panels are generally rated for output based on the sea level average energy. This is important to understand, since simply reading a panel's output specification can be deceiving. If two panels have identical efficiency and one has a higher listed output, that panel is not necessarily better. Instead, the higher output is simply the result of the second panel being larger. It can sometimes be more cost effective to buy larger panels, but they are not inherently a better option.

Input and Conditions Determine Output

The rated wattage of your panels is determined under ideal conditions. Most homes won't experience a perfect 1000 W/m², however, which means that the actual output of your panels is likely to be lower. Your panel installer will take measurements on and around your home to determine the overall amount of energy (known as flux) that you can expect to receive on a typical day. This provides a baseline for the energy that your panels can produce, although obviously no panel even approaches 100% efficiency.

Of course, the solar flux that is used to rate panels is not the only test condition that is ideal. Panels are also tested under ideal temperature conditions, which are 77F (25C). The warmer a panel is, the more efficiency it loses. This means that the rated efficiency of a panel will fall as it heats up. Most manufacturers will provide a separate operating efficiency, which is the efficiency you can expect from the panel when the sun has fully warmed it up. If your home is in an area with unusually high ambient temperatures, then you can expect to lose a bit more efficiency.

Putting It All Together

Although this may seem like a large number of variables to take in, your panel installer will work with you to calculate these values and use them to come up with a plan of action. Arming yourself with this knowledge beforehand will help you choose the right quantity and type of panels to meet your home's energy needs under all conditions.